1. Field of the Invention
When a plurality of terminals simultaneously use an acknowledgement/negative acknowledgement (ACK/NACK) channel in a wireless communication system, code division multiplexing (CDM) may be used to allow for the plurality of terminals. In CDM, each of the plurality of terminals transmits a signal multiplied by a spreading code allocated thereto.
The present invention relates to code hopping for efficiently mitigating interference among terminals in the same cell and between terminals of adjacent cells when each of a plurality of terminals uses a spreading code along a frequency axis and a spreading code along a time axis.
The present invention is derived from a research project partly supported by the Information Technology (IT) Research & Development (R&D) program of the Ministry of Information and Communication (MIC) and the Institute for Information Technology Advancement (IITA) [2005-S-404-13, Development of Radio Transmission Technology for 3G Evolution].
2. Description of the Related Art
The present invention relates to a method of transmitting an acknowledgement/negative acknowledgement (ACK/NACK) signal by a terminal as a response to data received from a base station.
A receiver sends an ACK signal to a transmitter when the receiver is successful in demodulating received data, and sends a NACK signal to the transmitter when the receiver is unsuccessful in demodulating the received data. Each of the ACK/NACK signal is expressed as one bit per codeword. A plurality of terminals should be able to simultaneously transmit their ACK/NACK signals by using given time and frequency resources through multiplexing.
Multiplexing techniques are classified into frequency division multiplexing (FDM) and code division multiplexing (CDM). While FDM is a form of multiplexing where a plurality of terminals use different time and frequency resources, CDM is a form of multiplexing where a plurality of terminals use the same time/frequency resources but transmit signals multiplied by orthogonal codes so as for a receiver to distinguish the plurality of users.
In uplink, a Zadoff-Chu sequence is widely used because it has an ideal peak to average power ratio (PAPR). Such a Zadoff-Chu sequence achieves orthogonality between terminals through cyclic delay without multiplying a signal by a specific code in a frequency domain.
A terminal transmits an uplink ACK/NACK signal to a base station to signify successful or unsuccessful receipt of downlink data. The uplink ACK/NACK signal requires one bit per codeword used to transmit the downlink data.
FIG. 1 illustrates time/frequency resources used by a terminal to transmit an uplink ACK/NACK signal through a control channel in a 3rd generation partnership projection long term evolution (3GPP LTE) system. Referring to FIG. 1, resources used by one control channel are grouped into two separate resource blocks. Each of the two resource blocks includes N subcarriers along a frequency axis, and 7 orthogonal frequency division multiplexing (OFDM) symbols, which corresponds to one slot, along a time axis. One slot has a length of 0.5 ms.
In FIG. 1, a plurality of terminals may commonly use one control channel. That is, one control channel may be shared by the plurality of terminals.
In this case, in order to distinguish the plurality of terminals using the same control channel, a specific code sequence is allocated to each of the plurality of terminals. That is, each of the plurality of terminals forms and transmits a signal spread on a frequency axis and a time axis by using its own specific code.
FIG. 2 illustrates a code sequence and a symbol transmitted to each of N subcarriers in an ACK/NACK channel occupying a resource block that includes the N subcarriers on a frequency axis and 7 OFDM symbols on a time axis. In FIG. 2, the resource block corresponding to one slot described with reference to FIG. 1 occupies N subcarriers on a frequency axis and includes 7 symbol blocks BL #0 through #6 on a time axis.
When CDM is used in order to distinguish signals of a plurality of terminals, a symbol and a sequence may be mapped to each time/frequency resource as shown in FIG. 2. In order to distinguish the plurality of terminals, a sequence is applied to each of the frequency axis and the time axis. In FIG. 2, a reference signal is used for channel estimation, and pre-determined signal is communicated between a terminal and a base station.
The base station estimates a channel by using a reference signal, and demodulates an ACK/NACK symbol transmitted by a control signal by using the estimated channel. Each time/frequency resource transmits a signal multiplied by two or three symbols.
That is, a time/frequency resource on which a reference signal is transmitted is obtained by multiplying a frequency-axis sequence symbol Cqm(k) a time-axis sequence symbol Ri (i=0, 1, 2). A time/frequency resource on which a control signal is transmitted is obtained by multiplying a frequency-axis sequence symbol Cqm(k), a time-axis sequence symbol Ci (i=0, 1, 2, 3), and an ACK/NACK symbol Q.
In FIG. 2, the frequency-axis sequence symbol Cqm(k) is given by Equation (1).
                                                        C              q              m                        ⁡                          (              k              )                                =                      exp            ⁡                          [                              ⅈ                ⁢                                                      2                    ⁢                    π                                                        N                    ZC                                                  ⁢                                  m                  ⁡                                      (                                                                                            (                                                      k                            -                            q                                                    )                                                ⁢                                                  (                                                      k                            -                            q                            +                            1                                                    )                                                                    2                                        )                                                              ]                                      ,                  k          =          0                ,        1        ,        2        ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            
In Equation (1), NZC is the length of a Zadoff-Chu sequence applied to a kth subcarrier on the frequency axis. The small letter m is a primary index, and q is a cyclic delay index.
One sequence is applied to each of a reference signal and a control signal along the time axis. That is, a sequence applied to a control signal in FIG. 2 is expressed as C0, C1, C2, C3. A sequence applied to a reference signal is expressed as R0, R1, R2.
Currently, in 3GPP LTE, three reference signals per slot are used for an uplink ACK/NACK channel.
Also, in order to distinguish terminals, a Zadoff-Chu sequence along a frequency axis is used and a discrete Fourier transformation (DFT) vector, a Walsh-Hadamard sequence, or a Zadoff-Chu sequence along a time axis may be used.